Recently, polyorganosiloxanes are attracting attention as a material having good light permeability, good heat resistance, low gas permeability and good chemical stability. Polyorganosiloxanes having various properties are produced by changing types of siloxane monomers, their ratio and reaction conditions in the production process and, accordingly, have been put to practical use in the various fields.
It is known that a polyoraganosiloxane having one or more alkoxysilyl groups and one or more hydrosilyl groups in the molecule has various applications and is very useful as an adhesion improving agent for a silicone elastomer adhesive composition, as described in Japanese Patent Application Laid-Open No. H05-194930, an adhesive component in a primer, as described in Japanese Patent Application Laid-Open No. H11-100550, and an intermediate of a polyorganosiloxane modified with various organic groups, as described in Japanese Patent Application Laid-Open No. H10-106511, as the hydrosilyl group may undergo a hydrosilation and the alkoxyl group may undergo a hydrolytic condensation reaction.
Polyorganosiloxanes are generally prepared by a hydrolysis and condensation reaction of chlorosilane and/or an alkoxysilane in contact with a stoichiometric amount of water in an organic solvent and in the presence of an acid or base catalyst. However, in a case where the polyorganosiloxane having a hydrosilyl group is prepared in this method, the hydrosilyl group is also hydrolyzed. Therefore, this method is not suitable.
Examples of the other general methods for preparing a polyorganosiloxane include a method where an organic silicon compound having a silanol group, i.e., —SiOH, condensation reacts with each other; a method where an organic silicon compound having a silanol group, i.e., —SiOH, condensation reacts with an organic silicon compound having an alkoxysilyl group, i.e., —SiOR; and a method where an organic silicon compound having an alkoxysilyl group, i.e., —SiOR, condensation reacts with each other, wherein R represents an alkyl group or an alkoxyalkyl group. In the aforesaid condensation reactions, an amount of a silanol group remaining in the polyorganosiloxane obtained is small. However, these methods need chemically intense catalysts to cause condensation reaction, such as, for instance, strong acids such as sulfuric acid and hydrochloric acid; strong bases such as sodium hydroxide, potassium hydroxide and tetramethylammonium hydroxide; and Lewis acids. If the aforesaid catalysts are used, a siloxane bond (Si—O—Si) is cut to cause rearrangement during the reaction and, therefore, the polyorganosiloxane obtained has random structures. Further, this method causes hydrolysis of the hydrosilyl group. Further, it is difficult to control the amount of an alkoxysilyl group in the polyorganosiloxane to leave some alkoxysilyl groups in the polyorganosiloxane.
Japanese Patent Application Laid-Open No. H02-235933, Patent Literature 4, describes that a silanol-containing siloxanes is condensated in the presence of sodium or potassium borate or phosphate as a catalyst to prepare an organosilicone condensate. Japanese Patent Application Laid-Open No. H03-197486, Patent Literature 5, describes that a silanol-containing siloxane is condensated in the presence of a catalyst selected from the group consisting of hydroxides, chlorides, oxides and basic metal salts of an alkali metal or an alkaline earth metal to prepare a polyorganosiloxane. Japanese National Phase Publication No. 2006-508216, Patent Literature 6, describes that even hydroxides of magnesium or calcium can work as a catalyst in condition of the presence of a protonic solvent to promote a condensation reaction between a silanol-containing siloxane and an alkoxysilane. Japanese National Phase Publication No. 2010-506982, Patent Literature 7, describes that a silicon-containing compound having a silanol group and/or an alkoxysilyl group reacts in the presence of a catalyst selected from the group consisting of strontium oxide, barium oxide, strontium hydroxide, barium hydroxide and a mixture thereof to prepare an organosilicone condensate.
In the methods described in the afore-mentioned Patent Literatures 4 to 7, rearrangement of the polyorganosiloxane chain is minimized and, thus, a polyorganosiloxane having a controlled structure is obtained. Further, these methods have an advantage that the catalyst is easily separated from the obtained polyorganosiloxane by filtration because these catalysts are solid. These advantages are favorable particularly in fields where accurate control on materials is required and any remaining impurity is untolerable, for instance, the fields of optical materials, electronic materials and medical materials.